A virtue of optical interferometers applied for studying objects with the use of short coherence optical radiation is a potential for acquisition of images of turbid media with high spatial resolution as well as noninvasive diagnostics in medical studies and non-destructive control in diagnostics of various equipment.
Optical interferometers being part of low coherence reflectometers and devices for optical coherence tomography are well known (see, for example, U.S. Pat. Nos. 5,321,501; 5,383,467; 5,459,570; 5,582,171; 6,134,003; International application No. WO 00/16034). Sometimes the optical interferometer is fully or partially implemented by using bulk optic elements (U.S. Pat. No. 5,383,467), but more often optical interferometers for these applications are made fiberoptic (U.S. Pat. Nos. 5,321,501; 5,459,570; 5,582,171).
The optical interferometer is typically designed as a Michelson interferometer (see X. Clivaz et al., High Resolution Reflectometry in Biological Tissues, Optics Letters, vol. 17, No. 1/Jan. 1, 1992, and also J. A. Izatt, J. G. Fujimoto et al., Optical Coherence Microscopy in Scattering Media, Optics Letters, vol. 19, No. 8/Apr. 15, 1994, p.590–592) or as a Mach-Zehnder interferometer (see J. A. Izatt, J. G. Fujimoto et al., Micron-Resolution Biomedical Imaging with Optical Coherence Tomography, Optics & Photonics News, October 1993, vol. 4, No. 10, p.14–19, and also U.S. Pat. No. 5,582,171). Regardless of the specific design used, an optical interferometer typically comprises a short coherent length light source, one or two beam splitters, sample and reference arms, and at least one photodetector. The sampling arm includes, as a rule, an optical measuring probe, the end of the reference arm being provided with a reference mirror (see A. Sergeev et al., In Vivo Optical Coherence Tomography of Human Skin Microstructure, Proc. SPIE, v. 2328, 1994, p. 144, and also X. J. Wang et al., Characterization of Human Scalp Hairs by Optical Low Coherence Reflectometry. Optics Letters, vol. 20, No. 5, 1995, pp. 524–526).
In a Michelson interferometer the sample and reference arms are bi-directional with a reference mirror placed at the end of the reference arm. For performing in-depth scanning the reference mirror either is connected to a device for moving said mirror mechanically (U.S. Pat. Nos. 5,321,501; 5,459,570), or its position is fixed and the in-depth scanning is performed with a piezoelectric scanning element (RU Pat. No. 2,100,787), or with a dispersion-grating optical delay line (K. F. Kwong, D. Yankelevich et al., 400-Hz Mechanical Scanning Optical Delay Line, Optics Letters, vol. 18, No. 7, Apr. 1, 1993).
The Michelson interferometers shown in background art, all execute intrinsically the same method for studying a sample (see U.S. Pat. Nos 5,321,501; 5,383,467; 5,459,570; RU Pat. No. 2,148,378). According to this method, a low coherent optical radiation beam is split into two beams. One of the beams is directed towards a sample along a sampling optical path and focused on the sample, while the other one is directed along a reference optical path. The low coherent optical radiation that passed along the sampling optical path in a forward and backward direction is combined with the low coherent optical radiation that passed along the reference optical path in a forward and backward direction, both the sampling and reference paths being bi-directional. Then the intensity of the low coherent optical radiation, which having passed along the sample path carries information about the sample, is visualized using a signal of interference modulation of the intensity of the optical radiation, which is a result of said combination.
The major drawback of Michelson interferometers, as well as of the executed method, is the low efficiency of use of optical source power. Even with the optimal splitting ratio in the coupler, which is 0.5 for a reciprocal configuration, a substantial portion of the input power is wasted in the reference arm and in the back way from the coupler/splitter to the light source. In addition, the optical radiation in the mentioned back way, contains an AC component representing the useful interference signal, whose amplitude is proportional to the one detected by the photodetector. This component could be used in another interferometer configuration to improve the signal-to-noise ratio (SNR), but is wasted in the Michelson interferometer and moreover, negatively impacts most known sources of a broadband optical radiation (e.g., semiconductor superluminescent diodes, doped-fiber amplified spontaneous emission superlums, solid state and fiberoptic femtosecond lasers).
A Mach-Zehnder interferometer doesn't return any substantial optical power back to the source and is more flexible in power splitting and coupling between arms because two different couplers are used for splitting optical radiation between reference and sampling arms and for combining optical radiation. Also, the second optical channel containing an anti-phase AC interference component is easily available and typically used in a differential detection arrangement to improve the SNR. Since it has unidirectional arms, it requires an optical circulator in the sampling arm to work with reflective samples (which is so far the only practical way to use low-coherence interferometry for biotissue imaging because penetration depth is fundamentally limited by light scattering to 2–3 mm and human and animal tissues and organs are much thicker). The most natural way for in-depth scanning with the Mach-Zehnder interferometer is with an in-line (transmissive) delay line, for example, a piezofiber optical delay. It is more common in the art to use reflecting delay lines based on a moving mirror, diffraction grating line, rotating mirrors, prisms, cams, and helicoid mirrors, but with the expense of another optical circulator added into the reference arm. In comparison with the Michelson interferometer, a transmissive delay line can only provide 50% of optical path modulation (and therefore 50% scanning depth) for the same delay element because of unidirectional rather than bi-directional operation.
Mach-Zehnder interferometers execute a slightly different method described, for example, in U.S. Pat. No. 6,485,413 and International application No. WO 00/16034. According to this method, a low coherent optical radiation beam is split into two beams. One of the beams is directed towards a sample along a first part of a unidirectional sampling optical path and focused on the sample. The other beam is directed along a unidirectional reference optical path. The low coherent optical radiation that carries information about the sample is directed along a second part of the unidirectional sampling optical path with the help of an additional optical system. Then optical radiations, which passed along the sampling and reference optical paths in a forward direction, are combined. The intensity of the low coherent optical radiation, which having passed along the sampling path and carries information about the sample, is visualized using a signal of interference modulation of the intensity of the resulting combined optical radiation. This method coupled with the Mach-Zehnder interferometer provides highly efficient use of the source power.
In-depth scanning of the sample in both methods can be implemented by changing the difference in the optical lengths of the optical paths for the first and the second optical radiation beams. Lateral scanning of the sample may also be carried out by the invention.
A hybrid method for studying a sample is known from International application No. WO 00/16034. According to this method, a low coherent optical radiation beam is split into two beams. One of the beams is directed towards a sample along a first part of a unidirectional sampling optical path, while the other beam is directed along a bi-directional reference optical path. The low coherent optical radiation that carries information about the sample is then directed along a second part of the unidirectional sampling optical path with the help of an additional optical system. Then optical radiations, which passed along the unidirectional sampling path in a forward direction and along the bi-directional reference optical path in a forward and backward direction are combined and the intensity of the low coherent optical radiation, which having passed along the sampling path carries information about the sample, is visualized using a signal of interference modulation of the intensity of the optical radiation, which is a result of said combination.
The hybrid interferometer described in the same International application No. WO 00/16034 comprises a low coherent length light source, two beam splitters, sample and reference arms, and at least one photodetector. The low coherent length light source is connected to the first port of the first beam splitter. The sampling arm is unidirectional and comprises two parts, the first part being connected to the second port of the first beam splitter. The first part of the sampling arm is provided with a probe, the latter including an optical system for focusing the first beam of low coherent optical radiation on the sample. The second part of the sampling arm is provided with an additional optical system for collecting light from the sample and taking the optical radiation, which carries information about the sample, to the first port of the second beam splitter. The third port of the first beam splitter is connected with the third port of the second beam splitter. The fourth port of the first beam splitter is connected to the reference arm, which is bi-directional, the distal end of the reference arm being provided with a reference mirror. A respective photodetector is connected to the second and fourth ports of the second beam splitter.
A drawback of the hybrid method, as well as the hybrid interferometer described in the International application No. WO 00/16034, is the necessity of an additional optical system for collecting light carrying information about the sample. The two optical systems (illuminating and collecting light) must be aligned and synchronously targeted to the same point in the sample with micron accuracy (the typical beam diameter at the tissue surface should be 15–30 μm), since even one beam diameter mismatch will lead to a complete loss of optical power collected by the second part of the sampling arm. In addition, some portion of the reference arm power inevitably returns back to the source, which may negatively impact the source performance.
A method and interferometer which makes highly efficient use of an optical power source, provides optimal signal-to-noise ratio, is simplified in use and more cost-effective is therefore desirable and provided by this invention.